This invention relates to a system for controlling a safety device for a vehicle, such as an air bag and a seat belt holding device.
Much attention has now been directed to an air bag as one effective means for protecting passengers against an impact due to collision of a vehicle. A control system for the air bag electrically detects the impact generated upon collision of the vehicle, and supplies electric current to a squib if the impact is above a predetermined level, thereby expanding the air bag.
Japanese Laid-Open (Kokai) Patent Application Nos. 55031/74 and 241467/88 disclose analog-type control systems. Such control system includes one acceleration sensor mounted on a vehicle. An analog deceleration signal from this acceleration sensor is fed to an integrating circuit where it is integrated, and this integral value is compared with a reference level at a comparator circuit. When the integral value exceeds the reference level, the comparator circuit outputs a collision decision signal to turn on a switching element, connected in series with a squib, to thereby supply electric current from a power source to the squib. In this conventional analog-type control system, however, the precision of the collision judgment or decision is not so high, and besides there is a possibility that due to oscillation, the collision decision signal may be outputted accidentally.
There is also known a control system in which the calculation of the above integral value as well as the comparison between the integral value and the reference value is performed by a microcomputer. In this control system, the impact due to the collision can be detected more precisely as compared with the analog-type control system. However, there is a possibility that the program runs away to produce a collision decision signal when pulse-like noises are inputted.
As described above, the conventional control systems of both the analog type and the digital type have the possibility of temporary malfunction, and therefore it has been desired to further improve the reliability of the air bag system.
A control system disclosed in Japanese Laid-Open Patent Application No. 207755/88 includes two comparator circuits for comparing two deceleration signals, fed respectively from two acceleration sensors, with a reference level to output two collision decision signals, and two switching elements responsive respectively to the two collision decision signals so as to be turned on. Only when the two switching elements are both turned on, electric current is supplied to a squib, and therefore the possibility of accidental activation of the squib can be extremely lowered.